An exosome is a small vesicle with a membrane structure secreted from a variety of cell types. Exosomes reportedly have a diameter of about 30-100 nm. It was observed that exosomes were derived from specific intracellular parts called multivesicular bodies (MVBs) and released and secreted to the outside of cells, instead of being directly detached and released from plasma membranes, by means of an electron microscope. That is, when fusion between multivesicular bodies and plasma membranes occurs, vesicles are released to the outside of cells. These vesicles are called exosomes. Although a molecular mechanism for a formation process of such exosomes is not clearly determined, it is known that a variety of immunocytes including B lymphocytes, T lymphocytes, dendritic cells, platelets, macrophages, etc., tumor cells, and stem cells, as well as red blood cells, produce and secret exosomes during their lifespan. In particular, it is known that, since exosomes derived from stem cells contain nuclear components as well as receptors and proteins, exosomes play important roles in intercellular communication. In addition, since exosomes contain a relatively small amount of animal serum, compared to stem cells, the risk of symptoms (zoonosis) due to infection caused by animal serum can also be eliminated. Considering such characteristics of exosomes, it is anticipated that cytotherapy using exosomes can be a new paradigm to overcome the limitations of existing stem cell therapies.